1. Field of the Invention
The present invention relates to an image forming apparatus such as copying machine and printer, particularly to an image forming apparatus that is provided with a function of writing image on a recording medium such as photosensitive material by scanning a laser beam from a light source using a polygon mirror.
2. Description of the Related Art
In an electro-photographic image forming apparatus such as laser printer, image is formed through the processes including charging, exposure, development, transfer, and fixation.
To be concrete, by irradiating a modulated light beam (laser beam) according to an image data, a static latent image of the image to be formed is formed on the surface of the charged photosensitive drum. And this latent image is developed into a toner image by a developing unit. This toner image is transferred onto a transfer material (recording paper) by a transfer roller. And the toner image on the transfer material is then fixed by fixing heat in a fixing unit. Thus, the image is formed on the transfer material.
In fixing the toner image on a transfer material, moisture in the transfer material is absorbed by the fixing heat and consequently the transfer material shrinks. Particularly in case of forming an image on both sides, the image size on the top surface becomes different from that on the back surface due to the shrinkage of the transfer material in fixing the top surface image, and there arises a problem that the top and back surface images become out of register (out of top/back register).
This can be coped with by controlling the frequency of pixel clock as well as controlling the speed of polygon mirror.
In controlling the speed of the polygon mirror in a color image forming apparatus, the phase of the polygon mirror is also controlled to adjust the color register after the speed control of the polygon mirror. That is to say, in case a color image is formed by using multiple polygon mirrors for each color, it is necessary to match the phase of the polygon mirrors with each other to eliminate color difference.
A prior art relating to this speed control and phase control of the polygon mirror is, for example, one in the Japanese Patent Application No. 2003-262991.